It is known in the art to employ variable camshaft timing (VCT) systems in internal combustion engines for improved fuel economy, emissions, and performance. VCT systems operate to vary the relative phasing between a camshaft and a crankshaft to optimize the cam timing over the range of engine operation.
An example of a VCT is a dual oil feed vane-type VCT. A dual oil feed vane-type variable cam timing unit provides an inner member or hub that is fixably connected to an end face of a camshaft. The hub has a series of vanes which are captured in cavities or pressure chambers provided in an outer member which is concentrically mounted on the hub. The outer member incorporates a camshaft timing pulley which is powered by the crankshaft via a belt which is looped over the camshaft pulley and a crankshaft timing gear. The vanes circumferentially divide the pressure chambers into an advance side and a retard side. A spool valve, fluidly communicative with the pressure chambers via the inner member and the camshaft, controls the fluid pressure in the advance side and retard side of the pressure chambers. Hence, by controlling the fluid in the advance and retard pressure chambers, the angular position of the timing pulley versus the crankshaft can be varied.
A disadvantage of such a VCT utilizing oil pressure and flow to control the phase of the camshaft is that the VCT response rate is dependent on the oil temperature and engine speed in order to achieve desired fuel economy and emission benefits.
The inventor herein has developed a system that improves variable camshaft timing systems and ameliorates the above problem.